Simplify a ratio, find a missing value and share an amount in proportion.
Works with whole numbers and decimals. Decimal ratios are multiplied up to whole numbers before they are simplified.
Ratios turn up everywhere, from scaling a recipe and mixing paint to splitting a bill or reading a map scale. This ratio calculator simplifies any two part ratio, works out a missing value and shares an amount in proportion, all at once. It reduces each ratio using the highest common factor of both numbers and shows the working, so it doubles as a quick ratio simplifier for homework checks.
Quick answer: A ratio calculator simplifies A : B by dividing both sides by their highest common factor, so 12 : 18 becomes 2 : 3 because the HCF is 6. To find a missing value in A : B = C : ?, use ? = B × C ÷ A.
In This Guide
What Is a Ratio?
A ratio compares two quantities, written as A : B. A ratio of 2 : 3 means that for every 2 parts of the first thing there are 3 parts of the second, so the whole is made of 5 equal parts. Ratios describe proportions, not amounts, which is why 12 : 18 and 2 : 3 mean exactly the same thing.
To simplify a ratio, divide both sides by their highest common factor (HCF). The HCF of 12 and 18 is 6, so 12 : 18 becomes 2 : 3. To find a missing value in A : B = C : ?, use ? = B × C ÷ A. To share an amount, divide it by the total number of parts and multiply by each side.
How Do You Use the Ratio Calculator?
Step 1: Enter the Ratio
Type the two parts of your ratio into A and B, and decimals are fine, so 1.5 : 2.5 is multiplied to 15 : 25 and then simplified to 3 : 5. Type the two parts of your ratio into A and B. Decimals are fine, so 1.5 : 2.5 is multiplied to 15 : 25 and then simplified to 3 : 5.
Step 2: Scale the Ratio
Enter a new value for A and the calculator finds the matching B, keeping the same proportion. This is ideal for recipes: if a sauce uses 12 g of flour to 18 g of butter and you have 30 g of flour, you need 45 g of butter. The cups to grams converter helps when one of those quantities is given in cups, so both sides use the same unit.
Step 3: Share an Amount
Enter a total and it is split in the ratio A : B. Sharing £100 in the ratio 12 : 18 gives £40 and £60, because there are 5 parts in the simplified ratio and each part is worth £20.
Step 4: Read the Results
You will see the simplest form, the unit ratio (such as 1 : 1.5), the percentage split, the scaled ratio and the shares. If you would rather express one part as a decimal, the fraction to decimal calculator converts a fraction such as 2/5 into 0.4 and 40%, which is handy for reports.
What Do Common Ratios Look Like in Simplest Form?
| Ratio | Simplest form | Unit ratio | Split of 100 |
|---|---|---|---|
| 12 : 18 | 2 : 3 | 1 : 1.5 | 40 and 60 |
| 16 : 9 | 16 : 9 | 1.78 : 1 | 64 and 36 |
| 1920 : 1080 | 16 : 9 | 1.78 : 1 | 64 and 36 |
| 250 : 1000 | 1 : 4 | 1 : 4 | 20 and 80 |
| 1.5 : 2.5 | 3 : 5 | 1 : 1.67 | 37.5 and 62.5 |
For example, 12 : 18 simplifies to 2 : 3, 250 : 1000 to 1 : 4, and 1920 : 1080 to 16 : 9, the ratio used for widescreen televisions and monitors. Screen sizes are a classic example: a 1920 by 1080 display reduces to the familiar 16 : 9 ratio used for widescreen televisions and monitors. Squash concentrate, cement mixes and map scales such as 1 : 25,000 all work the same way.
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How we checked this tool: the Tools Veria Editorial Team built this tool by hand, checked it against the official rules or published standards where they apply, and tested it against worked examples. Results are estimates for planning, not personal financial or tax advice. Read our editorial policy and disclaimer, or report an error.
Frequently Asked Questions
How do you simplify a ratio?
Find the highest common factor of both numbers and divide each side by it. For 12 : 18 the HCF is 6, so the ratio simplifies to 2 : 3.
How do you find a missing number in a ratio?
For A : B = C : ?, multiply B by C and divide by A. For 12 : 18 = 30 : ?, the answer is 18 × 30 ÷ 12 = 45.
How do you share an amount in a ratio?
Add the parts of the ratio together, divide the amount by that total, then multiply by each part. £100 in the ratio 2 : 3 gives £40 and £60.
Can a ratio have decimals?
Yes. Multiply both sides by 10, 100 or more until they are whole numbers, then simplify. 1.5 : 2.5 becomes 15 : 25, which simplifies to 3 : 5.
What is a unit ratio?
A unit ratio has 1 on one side, such as 1 : 1.5. It makes it easy to see how many times bigger one part is than the other.
Can I simplify a ratio with three parts?
Yes, the method is the same: divide every part by the highest common factor of all of them. This calculator handles two part ratios, so work through three parts by hand or in pairs.